linux-kernel - Re: [PATCH kcsan 6/9] tools/memory-model: Expand the cheatsheet.txt notion of relaxed

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Date:   Wed, 2 Sep 2020 11:54:48 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     paulmck@...nel.org
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, stern@...land.harvard.edu,
        parri.andrea@...il.com, will@...nel.org, peterz@...radead.org,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com
Subject: Re: [PATCH kcsan 6/9] tools/memory-model: Expand the cheatsheet.txt
 notion of relaxed

On Mon, Aug 31, 2020 at 11:20:34AM -0700, paulmck@...nel.org wrote:
> From: "Paul E. McKenney" <paulmck@...nel.org>
> 
> This commit adds a key entry enumerating the various types of relaxed
> operations.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>  tools/memory-model/Documentation/cheatsheet.txt | 27 ++++++++++++++-----------
>  1 file changed, 15 insertions(+), 12 deletions(-)
> 
> diff --git a/tools/memory-model/Documentation/cheatsheet.txt b/tools/memory-model/Documentation/cheatsheet.txt
> index 33ba98d..31b814d 100644
> --- a/tools/memory-model/Documentation/cheatsheet.txt
> +++ b/tools/memory-model/Documentation/cheatsheet.txt
> @@ -5,7 +5,7 @@
>  
>  Store, e.g., WRITE_ONCE()            Y                                       Y
>  Load, e.g., READ_ONCE()              Y                          Y   Y        Y
> -Unsuccessful RMW operation           Y                          Y   Y        Y
> +Relaxed operation                    Y                          Y   Y        Y
>  rcu_dereference()                    Y                          Y   Y        Y
>  Successful *_acquire()               R                   Y  Y   Y   Y    Y   Y
>  Successful *_release()         C        Y  Y    Y     W                      Y
> @@ -17,14 +17,17 @@ smp_mb__before_atomic()       CP        Y  Y    Y        a  a   a   a    Y
>  smp_mb__after_atomic()        CP        a  a    Y        Y  Y   Y   Y    Y
>  
>  
> -Key:	C:	Ordering is cumulative
> -	P:	Ordering propagates
> -	R:	Read, for example, READ_ONCE(), or read portion of RMW
> -	W:	Write, for example, WRITE_ONCE(), or write portion of RMW
> -	Y:	Provides ordering
> -	a:	Provides ordering given intervening RMW atomic operation
> -	DR:	Dependent read (address dependency)
> -	DW:	Dependent write (address, data, or control dependency)
> -	RMW:	Atomic read-modify-write operation
> -	SELF:	Orders self, as opposed to accesses before and/or after
> -	SV:	Orders later accesses to the same variable
> +Key:	Relaxed:  A relaxed operation is either a *_relaxed() RMW
> +		  operation, an unsuccessful RMW operation, or one of
> +		  the atomic_read() and atomic_set() family of operations.

To be accurate, atomic_set() doesn't return any value, so it cannot be
ordered against DR and DW ;-)

I think we can split the Relaxed family into two groups:

void Relaxed: atomic_set() or atomic RMW operations that don't return
              any value (e.g atomic_inc())

non-void Relaxed: a *_relaxed() RMW operation, an unsuccessful RMW
                  operation, or atomic_read().

And "void Relaxed" is similar to WRITE_ONCE(), only has "Self" and "SV"
equal "Y", while "non-void Relaxed" plays the same rule as "Relaxed"
in this patch.

Thoughts?

Regards,
Boqun


> +	C:	  Ordering is cumulative
> +	P:	  Ordering propagates
> +	R:	  Read, for example, READ_ONCE(), or read portion of RMW
> +	W:	  Write, for example, WRITE_ONCE(), or write portion of RMW
> +	Y:	  Provides ordering
> +	a:	  Provides ordering given intervening RMW atomic operation
> +	DR:	  Dependent read (address dependency)
> +	DW:	  Dependent write (address, data, or control dependency)
> +	RMW:	  Atomic read-modify-write operation
> +	SELF:	  Orders self, as opposed to accesses before and/or after
> +	SV:	  Orders later accesses to the same variable
> -- 
> 2.9.5
>